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Rhizomania is one of the most damaging and widespread diseases in the major sugar beet (Beta vulgaris L. subsp. vulgaris) growing regions worldwide. The causal agent, Beet necrotic yellow vein virus, is transmitted by a plasmodiophorid vector, Polymyxa betae Keskin. In this study, the inhibition of BNYVV multiplication was evaluated, under field conditions, in six sugar beet transgenic events generated based on gene silencing of the major coat protein gene (CP21). Phenotypic criteria, including scoring for disease symptoms and an immunoassay for viral accumulation, indicated that the transgenic events were less susceptible to rhizomania and had lower virus titre rates than non-transgenic plants of the susceptible cultivar '9597'. Disease severity and virus titres were comparable to the resistant cultivar 'Dorothea', which carries the Rz1-resistance gene and was bred through traditional means. This study provides a proof of concept for the effectiveness of the gene-silencing mechanism for introducing rhizomania resistance in sugar beet at the field level. [ABSTRACT FROM AUTHOR]